Low-voltage-insensitive electro-pyrotechnic device

ABSTRACT

A low-voltage-insensitive electro-pyrotechnic device comprising electrical input terminals, a bridge element connected to the electrical input terminals, and a transient voltage suppressor connected between the bridge element and one of the electrical input terminals. The device may utilize one bidirectional TVS on one side of the bridge element or two unidirectional TVSs on opposite sides of the bridge element, and may also include one or more resistors and/or an RF filter between the input terminals and bridge element.

FIELD OF THE INVENTION

The present invention relates primarily to the field ofelectro-pyrotechnic devices, and more particularly, to alow-voltage-insensitive electro-pyrotechnic device.

BACKGROUND OF THE INVENTION

Conventional electric detonators comprise hotwire initiators insertedinto a metal shell with load charges. The hotwire initiator is directlyelectrically exposed to external electrical input or stimulus from thelegwires to the outside world, making it susceptible to stray electricalvoltage or current entering the legwires, leading to possibleinadvertent firing of the detonator. Such electric detonators arelikewise vulnerable to ESD strikes (either wire-to-wire orwire-to-shell), which can also lead to inadvertent firing. Since hotwireresistance is typically in the range of 1 to 5 Ohm, significant currentwill flow through the hotwire at voltages of 1 to 5V without ESDprotection. This makes electric detonators susceptible to low voltagesources such as watch batteries, etc.

SUMMARY OF THE INVENTION

A low-voltage-insensitive electro-pyrotechnic device according to anembodiment of the present invention comprises electrical inputterminals, a bridge element connected to the electrical input terminals,and a transient voltage suppressor (“TVS”) connected between the bridgeelement and one of the electrical input terminals. The device mayutilize one bidirectional TVS on one side of the bridge element or twounidirectional TVSs on opposite sides of the bridge element, and mayalso include one or more resistors and/or an RF filter between the inputterminals and bridge element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a detonator according to a preferredembodiment of a low-voltage-insensitive electro-pyrotechnic deviceaccording to the present invention, and having a single bidirectionalTVS.

FIG. 2 is a diagram of a suitable layout for the circuit board includedin the embodiment of FIG. 1.

FIG. 3 is an electrical schematic for the embodiment of FIG. 1.

FIG. 4 is a schematic for an alternative embodiment having a singlebidirectional TVS, but two parallel resistors rather than one.

FIG. 5 is a schematic for another alternative embodiment in which thereare two unidirectional TVSs rather than a single bidirectional TVS.

FIG. 6 is a schematic showing an embodiment that further incorporates anRF filter.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIGS. 1-3 depict a low-voltage-insensitive electro-pyrotechnic deviceaccording to a preferred embodiment of the present invention in whichthere is a single bidirectional TVS and a parallel resistor. As seen inFIGS. 1 and 3, a hotwire initiator 20 is secured in the detonator 10with an endplug 15, with a TVS 22 placed before the hotwire initiator 20and a parallel resistor 24 placed before the TVS 22 and between the twoinput terminals 26 from the legwires 28. FIG. 2 shows an example of acircuit board layout (with the positions where the TVS 22 and resistor24 sit on the board depicted in dashed lines) using a 5.0 mm×13.3 mmboard 13 that can be accommodated inside the metal shell 30 of a commonmining and blasting detonator as shown in FIG. 1, with contact pads 21provided for connection to the hotwire initiator 20.

The TVS 22 blocks voltage applied across the legwires 28 until theclamping voltage is reached, at which point the TVS 22 starts conductingcurrent to the hotwire initiator 20. The TVS 22 can be discrete axial orradial, preferably a surface mount component. The TVS 22 is preferablychosen to conduct above 100V, more preferably 180V and most preferablyabove 200V but below 450V, typical voltage of electric blastingmachines. Examples of such TVSs are smaj170ca, smaj190ca, p4sma200ca andp4sma220ca. With the TVS 22 consisting of a bidirectional p4sma200ca,the assembly should block any DC input voltage up to 200V and will onlystart conducting at voltages above that (regardless of polarity since itis bidirectional).

The parallel resistor 24 placed before the TVS 22 and between the twoinput terminals 26 from the legwires 28, mitigates ESD charge buildupand also facilitates an electronic assembly continuity check performedat the downholes (to ensure there is electrical continuity between thelegwires to the electric detonator below in the downholes). The value ofthis resistor can be between 1 kOhm to 100 kOhm, most preferably 25 kOhmto 75 kOhm. Alternatively, a combination of resistors in series orparallel can be used to achieve the desired final resistance (e.g., 1kOhm to 100 kOhm). For example, as shown in FIG. 4, two 150 kOhmresistors 25 can be employed in parallel to produce a circuit resistanceof 75 kOhm.

As shown in FIG. 5, an alternate embodiment could instead employ twounidirectional TVSs 23. Such an embodiment could also be used with asingle resistor or a combination of resistors in series or parallel (notshown).

Optionally, as shown in FIG. 6, an RF filter 32 (comprising a pi(LC)filter, RC, RL networks, ferrite beads or chips, or a capacitor) canalso be incorporated in the front of the assembly, to enhance blockingof higher frequency AC voltage such as from extraneous RF or EMIsignals.

Also, instead of a TVS, another means for upwardly shifting the all-firevoltage as measured at the input terminals could be used, such as acombination of (e.g., 100) forward voltage diodes (each producing a dropof 1 volt) connected in series between one of the input terminals andthe hotwire initiator.

In use, the detonator 10 is connected to an electric blasting machine(not shown) yielding an output of at least 250V from a 450 uF capacitivedischarge, such as an REO Model CDJ 450J blasting machine with output of500V 450 uF capacitance network. Upon arming and firing the blastingmachine, the surge in voltage and current to the hotwire initiator 20results in deployment of the detonator 10 within 20-30 us or .so.

Although the present invention has been described in detail in thecontext of a preferred embodiment of a detonator used in mining andblasting, one skilled in the art will appreciate that numerousvariations, modifications, and other applications are also within thescope of the present invention. For example, although an embodiment ofthe invention for use in mining and blasting has been described, theinvention could be used in other applications such as military, defenseand aerospace applications where a low-voltage-insensitiveelectro-pyrotechnic device is needed. By using a TVS as high as 300V to500V or even 1000V by having serially connected TVSs, it is possible tohave all-fire voltage shifted to similarly high-voltage ranges. Further,in automotive applications such as airbags, such alow-voltage-insensitive electro-pyrotechnic device (for example, using a45V TVS) can be used as an initiator that is impervious to typicalautomotive voltages of 12V to 42V. Thus, the foregoing detaileddescription is not intended to limit the invention in any way, which islimited only by the following claims and their legal equivalents.

1. A low-voltage-insensitive electro-pyrotechnic device, comprising: a)first and second electrical input terminals; b) a bridge elementconnected to said electrical input terminals; and c) a first transientvoltage suppressor connected in series between said bridge element andsaid first electrical input terminal.
 2. The device of claim 1, whereinthe device is a detonator.
 3. The device of claim 1, wherein the deviceis a detonator for use in mining and blasting.
 4. The device of claim 1,wherein said bridge element is a hotwire initiator.
 5. The device ofclaim 1, wherein said transient voltage suppressor is bidirectional. 6.The device of claim 1, further comprising a second transient voltagesuppressor connected between said bridge element and said secondelectrical input terminal, wherein said first and second transientvoltage suppressors are unidirectional.
 7. The device of claim 1,further comprising a resistor connected between said electrical inputterminals and said bridge element.
 8. The device of claim 1, furthercomprising two or more resistors connected between said electrical inputterminals and said bridge element.
 9. The device of claim 1, furthercomprising one or more resistors connected between said electrical inputterminals and said bridge element, and further comprising a secondtransient voltage suppressor connected between said bridge element andsaid second electrical input terminal, wherein said first and secondtransient voltage suppressors are unidirectional.
 10. The device ofclaim 1, wherein the device is a detonator for use in mining andblasting and further comprises at least one resistor connected betweensaid electrical input terminals and said bridge element.
 11. The deviceof claim 10, wherein said transient voltage suppressor has a clampingvoltage of at least 100V.
 12. The device of claim 10, wherein saidtransient voltage suppressor has a clamping voltage greater than 100Vand less than 450V.
 13. The device of claim 10, wherein said transientvoltage suppressor is a p4sma200ca.
 14. The device of claim 10, whereinsaid resistor is connected in parallel between said input terminals andsaid bridge element and has a resistance greater than 1 kOhm and lessthan 100 kOhm.
 15. The device of claim 14, wherein said resistor has aresistance greater than 25 kOhm and less than 75 kOhm.
 16. The device ofclaim 14, further comprising an RF filter.
 17. The device of claim 1,further comprising an RF filter.
 18. A low-voltage-insensitiveelectro-pyrotechnic device, comprising: a) first and second electricalinput terminals; b) a bridge element connected to said electrical inputterminals; and c) an upwardly voltage shifting means connected in seriesbetween said bridge element and said first electrical input terminal.19. The device of claim 18, wherein said upwardly voltage shifting meanscomprises at least one transient voltage suppressor.
 20. The device ofclaim 18, further comprising an RF filter.